The present invention relates to a method of controlling diaphragm excursion of an electrodynamic loudspeaker. The electrodynamic loudspeaker may for example be mounted in a substantially sealed enclosure or ventilated enclosure. The substantially sealed enclosure or a ventilated enclosure may form part of a portable communication device such as a smartphone or tablet. The electrodynamic loudspeaker may be utilized for sound reproduction purposes, e.g. as a receiver for producing sound by acoustic coupling to the user's ear, or as a loudspeaker for reproducing recorded music or for voice reproduction in teleconferencing applications.
It is of significant interest to accurately estimate and limit the diaphragm excursion or displacement of an electrodynamic loudspeaker in numerous sound reproduction applications and devices for example to protect the electrodynamic loudspeaker against mechanical damage. The accurate control or limiting of the diaphragm excursion is important to avoid the diaphragm or diaphragm assembly is driven above its maximum excursion limit. This may occur if the maximum excursion limit of the electrodynamic loudspeaker in question is exceeded and various kinds of transitory or permanent mechanical damage may be imparted to the electrodynamic loudspeaker leaving the latter temporarily or permanently inoperative. The mechanical damage may be caused by collision between movable loudspeaker components, such as the voice coil, diaphragm or voice coil bobbin, and a stationary component of the loudspeaker such as the magnetic circuit. Since electrodynamic loudspeakers generally are rather non-linear devices, in particular at high sound pressure levels, with a large number of complex non-linearities it has proved difficult to accurately estimate the diaphragm excursion by various kinds of predictive model based approaches. In addition, parameter values of linear loudspeaker parameters of the electrodynamic loudspeaker, such as mechanical compliance and voice coil resistance, also vary slowly but markedly over time and temperature which leads to further challenges in maintaining an accurate model of the electrodynamic loudspeaker. Hence, it is of significant interest and value to provide a relatively simple methodology for accurately estimating and limiting diaphragm excursion of the electrodynamic loudspeaker despite these challenges.
The present methodology of controlling the diaphragm excursion comprises dividing the audio input signal into at least a low-frequency band signal and a high-frequency band signal by a band-splitting network. The low-frequency band signal is applied to a diaphragm excursion estimator for determining the instantaneous diaphragm excursion based on the low-frequency band signal. The frequency band-splitting of the audio input signal before signal limiting is advantageous because the diaphragm excursion is much larger at and below a fundamental resonance frequency of the electrodynamic loudspeaker than above the fundamental resonance frequency. This relationship is opposite to the sound pressure level which is much higher above than below the fundamental resonance frequency of the loudspeaker. To provide good excursion control and therefore good loudspeaker protection without unduly restricting the maximum sound pressure level capability of the loudspeaker, the audio input signal is divided or split into the low-frequency and high-frequency band signals. A cross-over frequency of the band-splitting network may lie above the fundamental resonance frequency of the electrodynamic loudspeaker such that the high-frequency band signal lies above the fundamental resonance frequency. The low-frequency band signal may therefore lie at the fundamental resonance frequency and below the fundamental resonance frequency of the electrodynamic loudspeaker. In this manner, the low-frequency band signal may be separately limited to prevent excessive diaphragm excursion and protect the loudspeaker while the high-frequency band signal may remain essentially unlimited. Hence, the subjective loudness of the sound generated by electrodynamic loudspeaker remains relatively unaffected of the limitation of the diaphragm excursion.